Fix step_trait on recent nightly: implement forward/backward_overflowing

[bidord]

Context

rust-lang/rust#155114 added two required methods to Step trait, so --features step_trait no longer builds on current nightly.

Change

• Implement forward_overflowing / backward_overflowing. These wrap via the underlying type's *_overflowing plus masked_new. That's what the trait docs recommend.
• Add tests
• Add changelog entry

---

AI disclosure: Claude Code wrote the tests and changelog entry.

[danlehmann]

thank you! I'll upload a patch to fix the check documentation error shortly - we can ignore that one. for the semantic versioning failure we probably have to update the compiler that's being used to perform the check?

[danlehmann]

doc fix: #112

[danlehmann]

let me know if you'd like to fix the semantic version issue yourself - if not, I'm happy to take over this patch

[bidord]

I quickly ran 2 tests when I saw the failure with the 2 solutions I could think of:

1. Run cargo-semver-checks on nightly toolchain: bidord@b43663c
   The job is still failing: https://github.com/bidord/arbitrary-int/actions/runs/30384093554/job/90358897787
   My understanding is that cargo-semver-checks-action is now also trying to build the latest release of this crate (v2.1.1) on the nightly toolchain so it can compare both. So, in theory, that failure should disappear as soon as you publish a new release on crates.io. Would that be acceptable?
   Also important to take into consideration: according to cargo-semver-checks's FAQ, support for nightly Rust versions is on a best-effort basis. So it might lead to other complications in the future. You might want to avoid that.

2. Remove step_trait from cargo-semver-checks: bidord@9781e47
   This works right-away: https://github.com/bidord/arbitrary-int/actions/runs/30384165752/job/90359138721
   Of course, this means removing some checks from your CI. IMO that's okay if you consider step_trait to be a nightly-only feature.

You can take over this patch and decide what's best.

Or if you want me to push one of the above solutions to this PR's branch, let me know.